Classic is a fucking challenge. I'm 4 or 5 missions in and just got TPK'd when two thin men warped into the mission map behind me and got a nice fat firing angle on my entire squad. I ragequit, and probably will need to start again, WITHOUT the tutorial, which went on for entirely too long and led you by the goddamn nose too long too. In fact, I suspect that my frustration was at the tutorial taking 90 minutes to play through. Although warping enemies into superior firing positions is pretty much one of the most infuriating game "cheats" out there.
The defensive angles are a little funky too. Baddies would fire on me in angles I'd swear I was in full cover and it'd bypass cover entirely. I guess it was like right along the damn boarder.
Still, that means the AI is good if I'm cursing at the game on mission 5.
Edit: I also had my first altitude bug which on that same level which cost me my assault trooper. I moved him into cover and he jumped down a ledge instead.